Hammett equation

About: Hammett equation is a research topic. Over the lifetime, 1162 publications have been published within this topic receiving 26633 citations.

A survey of Hammett substituent constants and resonance and field parameters

Abstract: The Hammett equation (and its extended forms) has been one of the most widely used means for the study and interpretation of organic reactions and their mechanisms. Although the Hammett methodology has been criticized by theoreticians because of its empirical foundation, it is astonishing that u constants, obtained simply from the ionization of organic acids in solution, can frequently predict successfully equilibrium and rate constants for a variety of families of reactions in solution. Almost every kind of organic reaction has been treated via the Hammett equation, or its extended form. The literature is so voluminous and extensive that there is no complete review of all that has been accomplished. Hammett's success in treating the electronic effect of substituents on the rates and equilibria of organic reactions1P2 led Taft to apply the same principles to steric and inductive and resonance effects? Then, more recently, octanol/ water partition coefficients (P) have been used for rationalizing the hydrophobic effects of organic compounds interacting with biological systems? The use of log P (for whole molecules) or n (for substituents), when combined with electronic and steric parameters, has opened up whole new regions of biochemical and pharmacological reactions to study by the techniques of physical organic chemistry.sf3 The combination of electronic, steric, hydrophobic, hydrophilic, and hydrogen-bonding7 parameters has been used to derive quantitative structure-activity relationships (QSAR) for a host of interactions of organic compounds with living systems or parts thereof. The binding of organic compounds to proteins,8 their interaction with enzymess and with cellsloJ1 and tiasues,12 their inhibition of organelles,l' and as antimalarial^'^

Direct palladium-catalyzed C-2 and C-3 arylation of indoles: a mechanistic rationale for regioselectivity.

Abstract: We have recently developed palladium-catalyzed methods for direct arylation of indoles (and other azoles) wherein high C-2 selectivity was observed for both free (NH)-indole and (NR)-indole. To provide a rationale for the observed selectivity ("nonelectrophilic" regioselectivity), mechanistic studies were conducted, using the phenylation of 1-methylindole as a model system. The reaction order was determined for iodobenzene (zero order), indole (first order), and the catalyst (first order). These kinetic studies, together with the Hammett plot, provided a strong support for the electrophilic palladation pathway. In addition, the kinetic isotope effect (KIE(H/D)) was determined for both C-2 and C-3 positions. A surprisingly large value of 1.6 was found for the C-3 position where the substitution does not occur (secondary KIE), while a smaller value of 1.2 was found at C-2 (apparent primary KIE). On the basis of these findings, a mechanistic interpretation is presented that features an electrophilic palladation of indole, accompanied by a 1,2-migration of an intermediate palladium species. This paradigm was used to design new catalytic conditions for the C-3 arylation of indole. In case of free (NH)-indole, regioselectivity of the arylation reaction (C-2 versus C-3) was achieved by the choice of magnesium base.

The Hammett Equation

Abstract: Preface 1. The Hammett sigma rho relationship 2. Elucidation of reaction mechanisms 3. The separation of inductive, resonance and steric effects: application of the Hammett equation to aliphatic systems 4. Application of the Hammett equation to data other than side chain reactivities of substituted benzenes 5. Thermodynamic aspects of the Hammett equation Problem discussion References Index.